Thiosulfate accounted for a significant portion of the chemical oxygen demand (COD) (7 to 20 percent) and the biochemical oxygen demand (BOD) (14 to 41 percent) of the four oil shale process waters studied. Accurate measurement of the thiosulfate oxygen demand of retort water is critical in assessing its environmental impacts on receiving waters and in designing biological treatment systems to treat it. The contribution of thiosulfate to the COD of oil shale retort waters can be accurately measured in a standard COD test. The BOD of thiosulfate in retort water is more difficult to determine and may require the development of a special thiosulfate-acclimated seed. Thiosulfate recovery of a known thiosulfate spike ranged from 92 to 100 percent in the COD test and from 54 to 119 percent in the BOD test. Considerable variability in recovery was found between the process waters studied. When determining the BOD of oil shale process waters, care must be taken to insure that there is a viable population of thiosulfate-oxidizing bacteria.